Field of the Invention
The present invention relates to a charged particle beam transferring apparatus for manufacturing micro-devices such as a semiconductor device, an electro-magnetic lens to be used in said apparatus, and a method for manufacturing said electro-magnetic lens. The invention relates more particularly to a charged particle beam transferring apparatus having high throughput, an electro-magnetic lens having a high response speed (which is capable of, for example, varying its focal length at a high speed), and a method for manufacturing said electro-magnetic lens.
Related Background Art
An electro-magnetic lens (referred to as "magnetic lens" hereinafter) comprises a coil and a core having two magnetic poles. In this magnetic lens, the intensity of the magnetic field on the axis of the magnetic lens is varied by changing a current flowing through the coil, whereby the focal length of the magnetic lens is adjusted.
The core is made of a magnetic material. Particularly, it is made of soft iron or Permalloy (trade name--a nickel alloy having high magnetic permeability), because large magnetic stresses are applied to the magnetic poles.
An electron beam demagnification transferring apparatus and an ion beam demagnification transferring apparatus are known as charged particle beam transferring apparatuses, and both transferring apparatuses are objects of the present invention. However, an electron beam demagnification transferring apparatus will be described as a representative of the charged particle beam transferring apparatus.
In the electron beam demagnification transferring apparatus, a pattern transferred onto a wafer is divided to form a plurality of sub-fields on a mask. Transfer is carried out by emission of electron beams to each sub-field. The emission of electron beams to the sub-fields is achieved by continuous movement, in a predetermined direction, of a mask stage on which the mask is loaded, and by deflection of the beams with a first deflector in association with the continuous movement of the mask stage. The electron beams which have passed through the sub-fields are emitted onto predetermined portions of a photosensitive member (wafer) in the following way. The magnification of a bundle of the electron beams is reduced by two magnetic lenses. A stage on which the photosensitive member is loaded is moved continuously in the reverse direction to the moving direction of the master (mask) stage, and a second deflector is operated to deflect the beams in association with the continuous movement of the stage carrying the photosensitive member. In this way, the electron beams are emitted onto the predetermined portions of the photosensitive member. Since the optical paths from the sub-fields to the photosensitive member differ from sub-field to sub-field in this structure, the focal lengths of the paired magnetic lenses must be adjusted for the respective sub-fields. Thus, the electron beams are emitted from a predetermined sub-field after the focal lengths of the paired magnetic lenses have been adjusted.
The paired magnetic lenses are arranged along the optical axis. The focal lengths of the paired magnetic lenses are adjusted by changing the currents flowing through their coils.
Soft iron or Permalloy per se of which a magnetic lens core is made has a frequency characteristic of 20 Hz or so. Thus, the conventional magnetic lens has a problem in that it takes more than 50 msec to stabilize the magnetic field produced in the magnetic poles after the current flowing through the coil has been changed.
The current is varied according to the individual sub-fields in the conventional charged particle beam transferring apparatus so as to adjust focal length of the magnetic lens for each of the sub-fields, and the operator must wait until the magnetic field is stabilized every time the current is changed. Therefore, the conventional magnetic lens presents problems in that it takes a long time to stabilize the magnetic field and the throughput is lowered.